This invention is generally in the field of microencapsulation techniques, particularly for use in the delivery of drugs or other biologically active agents.
Matrix or solid reservoir-type drug delivery systems generally require uniform distribution of drug within the encapsulating material. Proteins can be incorporated into polymer matrices in solution form or as a dry powder. The micronization of proteins and drugs to form solid particles suitable for microencapsulation (e.g., particles having a size less than about 10 μm) has been achieved using a variety of approaches including milling, spray-drying, spray freeze-drying, and supercritical anti-solvent (SAS) precipitation techniques. While proteins are generally more stable in a lyophilized (dry) state than a hydrated state, it is often difficult to produce dry micronized (less than 20 μm) protein particulates. The particle size is critical to drug release kinetics of matrix type devices.
Various milling techniques are known. For example, in U.S. Pat. No. 5,952,008 to Backstrom et al. jet milling is used to reduce the particle size of proteins and polypeptides to produce particles smaller than 10 μm for inhalation administration. U.S. Pat. No. 5,354,562 to Platz et al. discloses solid particle aerosol formulations of polypeptide drugs made by lyophilizing solutions of the drugs which contain milling stabilizers that inhibit degradation of the drug during subsequent milling. The lyophilized drug is milled in fluid energy mills that have been fitted with abrasion resistant materials. The resulting particles are between 0.5 to 4 μm when milled at high pressure and between 4 to 15 μm when milled at low pressure. U.S. Pat. No. 5,747,002 to Clark et al. discloses jet milling of sodium chloride to produce particles with a size distribution smaller than 7 μm. Immediately following the milling, the micronized particles are vacuum dried to prevent aggregation. U.S. Pat. No. 4,151,273 to Riegelman et al. discloses a method for preparing a glassy solid matrix of a carrier and a drug, formed at elevated temperature either with or without added solvent. The matrix is rapidly chilled to form a solid mass and ground to a powder for oral administration in a capsule.
Methods employing supercritical conditions also are well known. For example, U.S. Pat. No. 5,043,280 to Fischer et al. discloses a method for making a pharmaceutical preparation with minimal solvent residue. The method involves introducing a solution (of a substance and a carrier) at a supercritical state into a spray tower to extract a solvent from the sprayed solution to form a sterile product containing the substance embedded in the carrier. U.S. Pat. No. 5,851,453 to Hanna et al. discloses an apparatus to co-inject supercritical fluid and a vehicle containing at least one substance (e.g., drug or protein) in solution or suspension, such that the dispersion and extraction of the vehicle occur simultaneously by action of the supercritical fluid. Microparticulates less than 10 μm are produced. U.S. Pat. No. 5,833,891 to Subramaniam et al. discloses particle precipitation and coating using near-or supercritical fluid conditions. A fluid dispersion with a continuous phase dispersant and a precipitable substance are contacted with supercritical fluid anti-solvent so as to generate focused high frequency anti-solvent sonic waves, which break up the dispersion into extremely small droplets and cause the precipitation of particles between 0.1 and 10 μm in size. U.S. Pat. No. 5,874,029 to Subramaniam et al. discloses using an atomizer nozzle to spray a solvent and solute into a supercritical anti-solvent to cause depletion of the solvent in the droplets and production of nanoparticles in the range of 0.6 μm in size. U.S. Pat. No. 5,639,441 to Sievers et al. discloses producing an aerosol of particles when a solute in solution is mixed with a supercritical antisolvent. The particles are in the size range of 0.1 to 6.5 μm.
Spray drying methods also are well known in the art. For example, U.S. Pat. No. 5,700,471 to End et al. discloses a process for the making fine particles of drug or dye by spray-drying coarse particle dispersions of solutions of the drug or dye at temperatures above the melting point of the active agent. The active agent in solution is mixed with a protective aqueous colloid solution (e.g., consisting of gelatin or lactose) in water heated above the melting point of the drug, resulting in a molten emulsion of the drug in water. The emulsion is spray-dried, resulting in free-flowing powders with particles sizes less than 1 μm. U.S. Pat. No. 5,855,913 to Hanes et al. and U.S. Pat. No. 5,874,064 to Edwards et al. disclose the preparation of aerodynamically light particles between 5 and 30 μm, prepared by spray-drying a therapeutic agent mixed with surfactants or with therapeutic agent mixed with biodegradable polymers. Kornblum, J. Pharm. Sci. 58(1):125–27 (1969) discloses spray drying pure drug for purposes of micronization to form spheres in the range of 1–20 μm, and subsequent compression of the spray-dried formulation to produce tablets. Numerous precipitation techniques are also known. For example, U.S. Pat. No. 5,776495 to Duclos et al. discloses the formation of solid dispersions created by co-precipitation via drying of at least one therapeutic agent in an organic solvent with a hydrophilic polymer carrier with at least some solubility in the organic solvent. U.S. Pat. No. 4,332,721 to Bemini et al. discloses a process for preparing a spironolactive by precipitation with water from a solution with organic solvents in the temperature range of 0 to 30° C. U.S. Pat. No. 5,800,834 to Spireas et al discloses the use of systems to produce free-flowing powders from liquid lipophilic drugs or from water-insoluble drugs. The drugs are dissolved in suitable non-volatile solvents and mixed with carrier materials, such as microcrystalline or amorphous cellulose, to produce particles in the size range of 0.01 to 5 μm, and then coated with very fine silica powders. U.S. Pat. No. 5,780,062 to Frank et al. discloses formation of small particles of organic compounds by precipitation in an aqueous medium containing polymer/amphiphile complexes. U.S. Pat. No. 5,817,343 to Burke discloses a method for forming polymer/drug microparticles by forming a polymer solution/insoluble drug mixture; removing solvent from the mixture to form a hard matrix containing the drug particles in polymer; and micronizing the matrix by fragmenting (e.g., grinding, milling) the matrix below the glass-transition point of the polymer.
Sonication is another technique employed to micronize particles. For example, U.S. Pat. No. 4,384,975 to Fong et al. discloses the preparation of microspheres by solvent removal using sodium oleate as the emulsifier. Micronization of core material by milling or ultrasonic probe sonication of solid drug particles in polymer solution is disclosed. Tracy, Biotechnol. Prog, 14:108–15 (1998) discloses atomizing growth hormone in solution using an ultrasonic nozzle, freezing the dispersed droplets in a slurry of frozen ethanol, and then lyophilizing to remove the non-solvent and harden the droplets. The resulting hollow spheres are further micronized by ultrasonic probe treatment to fragment the spheres, which fragments are then encapsulated.
These methods are not desirable for micronizing certain types of agents, such as proteins. For example, exposure to high temperatures and/or an aqueous/organic solvent interface is known to be detrimental to protein stability leading to denaturation. It would be advantageous to provide dry, micronized particles of proteins, and a method of making such particles which substantially avoids or minimizes denaturation of the proteins. It would also be advantageous to provide dry micronized particles having a small, uniform size.
It is therefore an object of the present invention to provide a method of making dry particles of a protein or other agent, which process provides for the stabilization of the agent and produces very small particles of a uniform size, with minimal or no aggregation.